Description: FIELD OF INVENTION:
The present invention relates to the field of coordination chemistry. More specifically, the present invention relates to the gas adsorption by a porous bimetallic metal-organic framework composite and method of preparation thereof for increased gas storage abilities.

BACKGROUND OF INVENTION:
Metal-organic frameworks (MOFs), as a novel type of hybrid/composite organic-inorganic crystalline microporous solids, have emerged as a research hotspot in recent years. Due to their favorable physico-chemical properties, such as large specific surface area, unsaturated metal sites in certain structures, easy functionalization, permanent porosity, adjustable molecular and pore size, and adequate stability, they are viewed as promising candidates for various applications, including gas storage, separation, sensing, and catalyzing.

A fundamental feature of MOFs is their porosity which provides space on the micro- and meso-scale for confining and exposing their functionalities. Not only do the design and synthesis of MOFs play an important role in achieving higher porosity and stability in MOFs, but the development of improved activation methods is also essential in enabling the access to their porosity. However, most research groups use N2 adsorption experiments to study the porosity of MOF materials because it is highly accessible and inexpensive. Although IUPAC recommends avoiding carbon dioxide (CO2) to analyze porous materials with polar surfaces due to the even stronger quadrupole moment of CO2 compared to N2, CO2 adsorption is sometimes applied to access the porosity of MOFs with small pore sizes (eg. less than 0.45 nm) when N2 and Ar molecules are hard to diffuse into the pore space.

Typically, MOF-based absorbents possess large voids in their lattices. However, this space is not fully utilized because of the weak dispersive forces inside these pores, which cannot retain small adsorbate molecules, such as H2.

EP 3 932 540 A2 discloses a synthesis of novel Zn(II)-based Metal Organic Frameworks having mixed organic ligands of 1,3,5-benzene tricarboxylic acid (BTC) and 2-methylimidazole (mlm) through a simple and economic solvothermal method. The synthesized MOFs has cuboids morphology having high surface area (1248 m2/g) capable of hydrogen adsorption at 25°C temperature and 100 bar pressures. The hydrogen adsorption capabilities of the novel MOF are 0.2 wt %.

US11420983B2 discloses a mixed metal MOFs (MM-MOFs) of BTC, M-Cu-BTC, where in M is Zn(II), Ni(II), Co(II), and/or Fe(II) may be made using post-synthetic exchange (PSE) with metal ions. Such MM-MOFs may be used in H2 storage, especially Ni(II) and Co(II) MM-MOFs. Selected metal exchanged materials can provide gravimetric H2 uptake around 1.63 wt % for Zn-Cu-BTC, around 1.61 wt % for Ni-Cu-BTC, around 1.63 wt % for Fe-Cu-BTC, and around 1.12 wt % for Co-Cu-BTC at 77 K upto 100 bar.

Int. J. Hydrog. Energy., 2014, 39, 14912-14917 discloses the fabrication of core-shell nanocrystals by incorporating microporous UiO-66 into mesoporous MIL-101. The hydrogen storage capacity exhibited by the resulting core-shell nanocrystals was 2.4 wt % at 77 K upto 1 bar.

ACS Omega., 2018, 3, 167-175 discloses core-shell ZIF-8@ZIF-67 and ZIF-67@ZIF-8-based zeolitic imidazolate frameworks (ZIFs) were synthesized solvothermally using a seed-mediated methodology. The synthesized core-shell ZIF-8@ZIF-67 and ZIF-67@ZIF-8 frameworks conferred enhanced H2 (2.03 and 1.69 wt %) storage properties at 77 K and 1 bar.

Separation and Purification Technology., 307, 2023, 122679 discloses Solvent-assisted linker exchange (SALE) technique, which was successfully applied for the synthesis of porous ZIF-11@ZIF-8 core-shell composite structure metal-organic framework (MOF). Gas adsorption measurements were carried out for CO2, N2, CH4, C2H6, and C2H4 gases at 298 and 328 K and equilibrium pressures up to 4 bar. The results revealed a remarkable rise (~100 %) in CO2 adsorption capacity of ZIF-11@ZIF-8 nanoparticles (8.21 mmol g-1), compared to the pristine ZIF-11 (4.35 mmol g-1) at 298 K.

Journal of Industrial and Engineering Chemistry. 2013, 19, 1583-1586 discloses MWCNTs@MIL-53-Cu) composite MOF. The adsorption isotherm of CH4 on MWCNT@MIL-53-Cu at ambient temperature (298 K) and different pressures in the range of 0-35 bar and it exhibits the CH4 storage capacities of 13.72 mmol g-1.

J. Mater. Chem. 2009, 19, 7362-7370 discloses Mg-MOF-74. The adsorption isotherm of CH4 on at ambient temperature (298 K) and different pressures in the range 3.5 MPa and it exhibits the CH4 storage capacities of 207 cm3/g.

Microporous Mesoporous Mater. 2016, 220, 21-27 discloses graphene based porous carbon. The adsorption isotherm of CH4 at ambient temperature (298 K) and different pressures in the range 3.5 MPa and it exhibits the CH4 storage capacities of 253 cm3/g.

Therefore, there is a need in the art to provide a porous bimetallic organic framework composite for gas storage and a method of preparation thereof that is cost effective, economical and industrially scalable.

SUMMARY OF THE INVENTION:
This summary is provided to introduce a selection of concepts, in a simplified format, that are further described in the detailed description of the invention. This summary is neither intended to identify key or essential inventive concepts of the invention nor is it intended to determine the scope of the invention.

The present disclosure relates to a porous bimetallic metal-organic frameworks and method for preparing of bimetallic metal-organic frameworks and their gas storage application. More particularly, porous bimetallic (Cr-BDC@Cu-BTC) MOFs, in which a Cu-1,3,5 benzene tricarboxylate (Cu-BTC) MOF is grown onto the surface of a Cr-1,4 benzene dicarboxylate (Cr-BDC) MOF to achieve improved surface properties and further contribute to the high gas adsorption capacity.

The present invention relates to a porous bimetallic metal-organic frameworks composite (Cr-BDC@Cu-BTC) for gas adsorption said composite comprising: Cr-1,4 Benzene dicarboxylate (Cr-BDC) and Cu-1,3,5 Benzene tricarboxylate (Cu-BTC).

The present invention also pertains to a method for preparation of porous bimetallic metal-organic frameworks for improved gas adsorption, wherein the method comprising:
a. preparing of Cr-BDC;
b. preparing of Cu-BTC;
c. contacting of the Cr-BDC and the Cu-BTC to obtain a reaction mixture;
d. sonicating the reaction mixture and incubating, to obtain a precipitates; and
e. drying the precipitates to obtain the porous bimetallic metal-organic frameworks.

The present invention also provides a process for gas adsorption, said process characterized in employing a porous bimetallic metal-organic frameworks composite.

DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
These and other features, aspects, and advantages of the present disclosure will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:

Figure 1 illustrates FESEM images of (a) & (b) Cr-BDC, (c) Cu-BTC, (d) Cr-BDC@Cu-BTC and the (e) FESEM-EDS elemental mapping images of porous bimetallic metal-organic frameworks composite (Cr-BDC@Cu-BTC).

Figure 2 illustrates XRD Patterns of Cr-BDC, Cu-BTC and synthesized porous bimetallic metal-organic frameworks composite (Cr-BDC@Cu-BTC).

Figure 3 illustrates N2 adsorption-desorption isotherms of Cr-BDC, Cu-BTC and synthesized porous bimetallic metal-organic frameworks composite (Cr-BDC@Cu-BTC) at 77 K.

Figure 4 illustrates Hydrogen Adsorption Isotherms of Cr-BDC, Cu-BTC and Cr-BDC@Cu-BTC at 298 K upto 100 bar.

Figure 5 illustrates Methane Adsorption Isotherms of Cr-BDC, Cu-BTC and Cr-BDC@Cu-BTC at 298 K upto 65 bar.

Figure 6 illustrates CO2 Adsorption Isotherms of Cr-BDC, Cu-BTC and Cr-BDC@Cu-BTC at 298 K upto 25 bar.

DETAILED DESCRIPTION OF THE INVENTION:
The present disclosure addresses the drawbacks of the art and provides for a bimetallic metal-organic frameworks composite with higher porosity for gas storage application. Further, the present invention also provides a method for preparing metal-organic frameworks. The method is environmentally friendly and requires less time as compared to other existing methods.

For the purpose of promoting an understanding of the principles of the disclosure, reference will now be made to the embodiment illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the disclosure is thereby intended, such alterations and further modifications in the illustrated system, and such further applications of the principles of the disclosure as illustrated therein being contemplated as would normally occur to one skilled in the art to which the disclosure relates. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skilled in the art to which this disclosure belongs. The system, methods, and examples provided herein are illustrative only and not intended to be limiting.

More specifically, any terms used herein such as but not limited to “includes”, “comprises”, “has”, “consists” and grammatical variants thereof is inclusive or open-ended and does not exclude additional, unrecited elements or method steps. The specification will be understood to also include embodiments which have the transitional phrase “consisting of” or “consisting essentially of” in place of the transitional phrase “comprising.” The transitional phrase “consisting of” excludes any element, step, or ingredient not specified in the claim, except for impurities associated therewith. The transitional phrase “consisting essentially of” limits the scope of a claim to the specified materials or steps “and those that do not materially affect the basic and novel characteristic(s)” of the claimed disclosure.

As used herein, the term “about” is used to indicate a degree of variation or tolerance in a numerical or quantitative value. It indicates that the disclosed value is not intended to be strictly limiting, and may vary by plus or minus 5%, without departing from the scope of the invention.

Unless otherwise defined, all terms, and especially any technical and/or scientific terms, used herein may be taken to have the same meaning as commonly understood by one having an ordinary skill in the art.

Reference is made herein to some “embodiments.” It should be understood that an embodiment is an example of a possible implementation of any features and/or elements presented in the attached claims. Some embodiments have been described for the purpose of illuminating one or more of the potential ways in which the specific features and/or elements of the attached claims fulfil the requirements of uniqueness, utility and non-obviousness.

As used herein the terms “method” and “process” have been used interchangeably.

The present disclosure relates to a porous bimetallic metal-organic frameworks composite for gas adsorption and method of preparation of the porous bimetallic metal-organic frameworks composite. The synthesized porous bimetallic metal-organic frameworks composite is cost effective, simple, efficient, environment friendly, and has increased gas storage abilities and requires lesser synthesis time as compared to other existing methods.

The present application describes a porous bimetallic metal-organic frameworks for gas storage application, wherein the adsorbed gases are hydrogen, methane and carbon dioxide.

The present invention provides a facile synthesis route for a novel Cr-BDC@Cu-BTC structure, containing two different MOFs, namely Cr-BDC and Cu-BTC with different topologies in the absence of any surfactant agents.

Further, the present invention provides a Cr-BDC@Cu-BTC that is microporous material and the pore-diameter for the synthesized MOF (Cr-BDC@Cu-BTC) is 1.8759 nm.

The present invention relates to a porous bimetallic metal-organic frameworks composite (Cr-BDC@Cu-BTC) for gas adsorption, said composite comprising: Cr-1,4 Benzene dicarboxylate (Cr-BDC) and Cu-1,3,5 Benzene tricarboxylate (Cu-BTC).

In one embodiment, the porous bimetallic metal-organic frameworks composite is Cr-BDC@Cu-BTC.

In another embodiment, the porous bimetallic metal-organic frameworks, wherein the porous bimetallic metal-organic frameworks have a pore-diameter in a range of 0.5 to 2.0 nm.

It yet another embodiment, the porous bimetallic metal-organic frameworks, wherein the porous bimetallic metal-organic frameworks comprise Cr-BDC and Cu-BTC in a ratio of about 1:1.

The present invention also pertains to a method for preparation of porous bimetallic metal-organic frameworks for improved gas adsorption, wherein the method comprising:
a. preparing of Cr-BDC;
b. preparing of Cu-BTC;
c. contacting of the Cr-BDC and the Cu-BTC to obtain a reaction mixture;
d. sonicating the reaction mixture and incubating, to obtain a precipitates; and
e. drying the precipitates to obtain the porous bimetallic metal-organic frameworks.

In one embodiment, there is provided the method, wherein, the sonication is carried out for 20 minutes to 40 minutes.

In another embodiment, there is provided the method, wherein, the heating of reaction mixture is carried out for 8 to 12 hours at a temperature in the range of 80 to 120°C.

In yet another embodiment, there is provided the method, wherein the precipitates are purified by solvent exchange method using solvent, selected from methanol, dimethyl formamide (DMF), diethyl formamide (DEF), acetonitrile (MeCN), ethanol, water or combination thereof.

In some embodiments there is provided the method, wherein the drying of precipitates is carried out in a vacuum for 10 to 15 hours at a temperature in the range of 60 to 80°C for 10 hours to 15 hours.

In some embodiments there is provided the method, wherein the porous bimetallic metal-organic frameworks are developed without the use of surfactant.

In one of the embodiments there is provided the method, wherein, the method of preparing the Cr-BDC comprising the steps of:
a) preparing a reaction mixture of Chromium (III) nitrate Cr(NO3)3·9H2O, benzene-1,4-dicarboxylic acid (H2bdc) and water with continuous stirring;
b) adding of additive acid to the reaction mixture of step a) and heating for 8 hours at 220 °C and then cooling slowly at room temperature;
c) centrifugating the reaction mixture of step (b) by adding DMF;
d) sonicating the reaction mixture obtained in step c) and centrifugating to obtain the precipitates; and
e) transferring, collecting, drying the precipitates and vacuum drying to obtain the Cr-BDC.

In another embodiment of the present invention, there is provided the method, wherein the method of preparing a Cu-BTC comprising the steps of:
a) preparing a reaction mixture by dissolving benzene-1,3,5-tricarboxylate (BTC), dimethylformamide (DMF) and alcohol;
b) dropwise adding of reaction mixture of step a) into the solution of Cu(NO3)2.3H2O and water with stirring for 10 to 15 minutes at room temperature to obtain a homogenous mixture;
c) sonicating the homogenous mixture obtained in step b) and heating to obtain the precipitates; and
d) filtering, collecting, drying the precipitates and vacuum activating to obtain the (Cu-BTC).

The present invention also provides a process for gas adsorption, said process characterized in employing a porous bimetallic metal-organic frameworks composite.

In one embodiment, there is provided a process for gas adsorption, wherein the gas is selected from the group comprising hydrogen, methane, and carbon dioxide.

In another embodiment, there is provided a process for gas adsorption, wherein the gas is hydrogen and is adsorbed is in a range of 0.1wt.% to 0.7 wt. % at a temperature of 298 K, pressure of 100 bar.

In a preferred embodiment, wherein the gas is hydrogen and is adsorbed is in a range of 0.4 wt.% to 0.6 wt % at a temperature of 298 K and at a pressure of 100 bar.

In another preferred embodiment, wherein the gas is hydrogen and is adsorbed is at 0.45 wt.%, 0.46% and 0.56 wt % at a temperature of 298 K and at a pressure of 100 bar.

In yet another embodiment, wherein the gas is methane and is adsorbed in a range of 100 cc/g to 300 cc/g at a temperature of 298 K and at a pressure of 65 bar.

In a preferred embodiment, wherein the gas is methane and is adsorbed in a range of 160 cc/g to 225 cc/g at a temperature of 298 K and at a pressure of 65 bar.

In another preferred embodiment, wherein the gas is methane and is adsorbed at 166 cc/g, 205 cc/g, and 219 cc/g at a temperature of 298 K and at a pressure of 65 bar.

In another embodiment, there is provided a process for gas adsorption, wherein the gas is carbon dioxide and is adsorbed in a range of 250 cc/g to 450 cc/g at temperature of 298 K, and at a pressure of 25 bar.

In a preferred embodiment, wherein the gas is carbon dioxide and is adsorbed in a range of 325cc/g to 400cc/g at temperature of 298 K, and at a pressure of 25 bar.

In another preferred embodiment, wherein the gas is carbon dioxide and is adsorbed at 339cc/g, 373 cc/g and 381cc/g at temperature of 298 K, and at a pressure of 25 bar.

The present invention provides that the properties of Cr-BDC and Cu-BTC are favourable for enhanced gas adsorption and storage.

Further, the present invention provides a porous bimetallic metal-organic frameworks composite (Cr-BDC@Cu-BTC) for gas storage wherein the Cr-BDC is a three-dimensional chromium terephthalate-based porous material with the empirical formula [Cr3O(OH)(H2O)2(bdc)3]. It possesses the framework of augmented MTN zeolite topology. Further, Cr-BDC contains hybrid super-tetrahedral units consisting of terephthalate ligands and trimeric chromium octahedral clusters. It possesses a neotype architecture with 1.2-1.6 nm pores, 2.7-3.4 nm mesoporous cages and multiple unsaturated Cr-sites, which acts as active adsorption sites. The molecular formula of Cr-BDC is C24H17O16Cr3 and the molecular weight is (Mw) is 717.4 g mol-1. Further, the air and moisture sensitivity are stable against water. The colour of Cr-BDC is green and it is powdered form.

Cu-BTC presents a three-dimensional porous framework formed by the coordination of copper cations (Cu2+) and benzene-1,3,5-tricarboxylate (BTC) linker molecules which form the dimeric copper paddle-wheel structural building blocks. Cu-BTC exhibits abundancy of active Cu sites and is formed by a 3-D assembly of Cu dimers of the Cu clusters and oxygen atoms of the benzene-1,3,5-tricarboxylic acid linker in Fm3¯m symmetry, which results in the formation of face-centered-cubic lattices as Cu2(COO)4 - paddle wheels. The molecular formula is C18H6Cu3O12 and the molecular weight is (Mw = 604.9 g mol-1). Further, the air and moisture sensitivity vary from sensitive to humid streams and the colour of Cu-BTC is Blue and it is powdered form.

In the present invention, the best method for synthesis of Cr-BDC@Cu-BTC MOF is hydrothermal method and solvent is water. The temperature used was 100°C and cooling or quenching conditions were at 30 °C per hour for 6 hours. Mineralizing agent used is HNO3 and metal ions are Cr, Cu and ligands used are benzene-1,3,5-tricarboxylic acid (BTC), benzene-1,4-dicarboxylic acid (BDC).

The additive acid/mineralizing agent (Nitric Acid (HNO3)) promote the nucleation process or interfere with the growth of particles due to the variation of the pH and solubility of the raw materials. HNO3 act as a mineralizing agent to increase the crystallinity of microporous materials and favours the formation of highly crystalline phases in MOFs.

EXAMPLES:
The present disclosure is further illustrated by reference to the following examples which is for illustrative purposes only and does not limit the scope of the disclosure in any way. These examples are not intended to be inclusive of all aspects of the subject matter disclosed herein, but rather to illustrate representative features, methods, compositions, and results. These examples are not intended to exclude equivalents and variations of the present disclosure, which are apparent to one skilled in the art.

Example 1: Preparation of component 1 (Cr-BDC)
Cr-BDC: The synthetic procedure for Cr-BDC. 400 mg of Chromium (III) nitrate Cr(NO3)3·9H2O and 164 mg of benzene-1,4-dicarboxylic acid H2bdc were mixed in 5 mL of H2O and stirred for 3 minutes followed by the addition of additive acid (i.e., HNO3). The reaction mixture is then transferred to the PTFE/Teflon liner in a hydrothermal autoclave and heated for 8 hours at 200-220 °C and then cooled slowly at room temperature preferably, at a rate of 30 °C h-1 for 6 hours. The reaction mixture of the autoclave was transferred to two centrifuge tubes and the supernatant solution was carefully removed after centrifugation. Further, water (5 mL) was added in each tube and the solid was evenly dispersed in the aqueous phase. After renewed centrifugation and removal of the supernatant solution, DMF (5 mL) was added to each tube which were placed in a hot (80 °C) ultrasonic bath and sonicated for 1 hour. Centrifugation was further carried out to separate Cr-BDC and DMF. The precipitate was obtained and transferred to 25 mL beaker and was stirred with 10 mL of water at 70 °C for 5 hours. After separation by centrifugation, the same washing procedure but using ethanol was repeated once more at the same temperature. The final product was obtained by centrifugation and dried in a vacuum oven at 80-120 °C for 5 hours.

Example 2: Preparing of component 2 (Cu-BTC)
BTC (2.12 g) was dissolved in a mixture of DMF (15 mL) and alcohol (15 mL), and the above mixture was dropwise added in the solution of Cu(NO3)2.3H2O (2.42 g) in ultrapure water (30 mL) and was stirred for 10 minutes at room temperature. The homogeneous mixture was placed in a 100 mL Teflon-lined stainless-steel autoclave and heated at 85-120°C for 12 hours. The blue product was collected by filtration, and washed thoroughly with fresh alcohol over three times, followed by solvent exchange process with chloroform solution over four times for 2 days. The activated Cu-BTC was obtained by vacuum activation of the product at 393 K for an additional 16 hours.

Example 3: Preparing porous bimetallic metal-organic frameworks through epitaxial growth
In order to prepare a porous bimetallic MOF, 1.0 g of the Cr-BDC, as seeding crystals, were put into the reaction solution of Cu-BTC, consisting of Cu(NO3)2·3H2O (1.03 gm) dissolved in water (7.5 ml), H3BTC (0.5 gm) is dissolved in DMF (7.5 ml) and ethanol (7.5 ml). The resulting mixture was sonicated for 30 minutes in an ultrasonic bath and thereafter transferred into a reaction vessel where it was heated up to 100°C and kept at that temperature for 10 hours. The resultant product was washed with methanol and kept drying in vacuum for 12 hours. Approximately 100 ml methanol was used for purification of 1 gm of final product. The physical properties and gas storage measurements of Cr-BDC, Cu-BTC and Cr-BDC@Cu-BTC is shown in table 1. At low pressures, the small pores <0.8 nm starts filling with methane. The porous bimetallic organic composite material has more small pores and thereby has higher uptake. As pressure increases, saturation of all pores take place providing an equilibrium saturation value. By increasing pressure, pores with a size of 1-2 nm start filling with methane. The presence of narrow mesopores of 2-3 nm leads to slower gas diffusion through the MOF composites and the adsorption slows down.
Table - 1: Physical properties and gas storage (H2, CH4 and CO2) capacities of Cr-BDC, Cu-BTC and Cr-BDC@Cu-BTC

S.
No Sample Name Surface
Area (m2g-1) Pore
Volume
(cm3g-1)
H2 Uptake
(Upto 100 bar at 298 K) wt %
CH4 Uptake (Upto 65 bar at 298 K) cc/cc CO2 Uptake (Upto 25 bar at 298 K) cm3/g(STP)
1. Cr-BDC 2833 1.2809 0.4656 205 373
2. Cu-BTC 1474 0.5972 0.4586 166 339
3. Cr-BDC@Cu-BTC 2334 1.0947 0.5548 219 381

Example 4: Characterization of porous bimetallic metal-organic frameworks
The morphology of the prepared samples was characterized by Field Emission Scanning Electron Microscope (FESEM) and XRD. The images of the samples are presented in Figure 1.

X-ray diffraction: Powder x-ray diffractometer (XRD) analysis (Pan Analytical, Netherland) was carried out with Cu-Ka radiation (? = 1.54 Å) as X-ray source in 2? range from 5 to 70? at a scanning rate of 2? min-1. The powder X-ray diffraction (XRD) peaks of Cr-BDC, Cu-BTC and CrBDC@CuBTC are shown in Figure 2. The intensity of the peaks at 7.1, 8.5, 9.1 and 13.2 for Cr-BDC were detected. For Cu-BTC, characteristic peaks appeared at 2? = 6.8°, 9.5°, 11.6°, 13.5°, 14.9°, 16.6°, 17.5°, 19.0°, 20.2°, 24.1°, 25.9°, 29.3°, 35.3° and 39.1° correspond to the (200), (220), (222), (400), (420), (422), (511), (440), (442), (551), (731), (751), (773) and (882) crystal planes respectively. The XRD pattern of the CrBDC@CuBTC indicates the co-existence of both MOFs (Figure 2).

Surface Area: N2 sorption isotherms were determined at 77 K. The specific surface area, pore size and total pore volume values are given in Table 1. N2 sorption isotherms were determined at 77 K on a BELSORP MAX II analyzer. The specific surface area, pore size and total pore volume values of the synthesized MOFs were calculated according to the Brunauer-Emmett-Teller (BET) technique via N2 adsorption-desorption at 77 K using a BELSORP MAX II instrument (BEL Japan, Inc.). Prior to performing the BET tests, all samples were degassed at 393 K for 12 h to remove any impurities.

Example 5: Comparative data for reaction condition used in the synthesis of MOF
To identify the effect of mineralizing agent in the formation of Cr-BDC, reactions have been carried out. Nitric acid gives high yield, high surface area and good pore diameter, which enhances the hydrogen uptake capacity as shown in Table 2.

Table 2:
Additive Yield (%) of final MOF SBET (m2g-1) Vpore (cm3g-1)
Hydrofluoric acid 47.4 3620 1.82
Hydrochloric acid 36.3 1560 0.79
Sulfuric acid 48.2 1750 0.81
Nitric acid 82.3 3450 1.66
Trifluoroacetic acid 73.8 2650 1.34
Phenylphosphonic acid 51.9 2460 1.49
Fumaric acid 28.7 760 0.69
Citric acid 37.2 740 0.58
Formic acid 27.1 590 0.56
Succinic acid 59.8 2510 1.28
Benzoic acid 39.4 1760 0.93
Acetic acid 24.4 2750 1.55

Gas adsorption measurements: The equilibrium gas adsorption isotherms for various pure gases, including H2, CH4 and CO2 on the prepared samples were obtained at different temperatures and pressures and provided in above mentioned Table-1 and Figures 4 - 6. , Claims:1. A porous bimetallic metal-organic frameworks composite(Cr-BDC@Cu-BTC) for gas adsorption, said composite comprising: Cr-1,4 Benzene dicarboxylate (Cr-BDC) and Cu-1,3,5 Benzene tricarboxylate (Cu-BTC).

2. The porous bimetallic metal-organic frameworks as claimed in claim 1, wherein the porous bimetallic metal-organic frameworks have a pore-diameter in a range of 0.5 to 2.0 nm.

3. The porous bimetallic metal-organic frameworks as claimed in claim 1, wherein the porous bimetallic metal-organic frameworks comprise Cr-BDC and Cu-BTC in a ratio of about 1:1.

4. A method for preparation of porous bimetallic metal-organic frameworks for improved gas adsorption, the method comprising:
a. preparing of Cr-BDC;
b. preparing of Cu-BTC;
c. contacting of the Cr-BDC and the Cu-BTC to obtain a reaction mixture;
d. sonicating the reaction mixture and incubating to obtain a precipitates; and
e. drying the precipitates to obtain the porous bimetallic metal-organic frameworks.

5. The method as claimed in claim 4, wherein, the sonication is carried out for 20 minutes to 40 minutes, wherein, the incubation of reaction mixture is carried out for 8 to 12 hours at a temperature in the range of 80 to 120°C.

6. The method as claimed in claim 4, wherein the precipitates are purified by solvent exchange method using solvent selected from methanol, dimethyl formamide(DMF), diethyl formamide (DEF), acetonitrile (MeCN), ethanol, water or combination thereof.

7. The method as claimed in claim 4, wherein the drying of precipitates is carried out in a vacuum for 10 to 15 hours at a temperature in the range of 60 to 80°C for 10 hours to 15 hours.

8. The method as claimed in claim 4, wherein the porous bimetallic metal-organic frameworks is developed without the use of surfactant.

9. The method as claimed in claim 4, wherein, a method of preparing the Cr-BDC comprising the steps of:
a) preparing a reaction mixture of Chromium(III) nitrate Cr(NO3)3·9H2O, benzene-1,4-dicarboxylic acid (H2bdc) and water with continuous stirring;
b) adding of additive acid to the reaction mixture of step a) and heating for 8 hours at 220 °C and then cooling slowly at room temperature;
c) centrifugating the reaction mixture of step (b) by adding DMF;
d) sonicating the reaction mixture obtained in step c) and centrifugating to obtain the precipitates; and
e) transferring, collecting, drying the precipitates and vacuum drying to obtain the Cr-BDC.

10. The method as claimed in claim 4, wherein a method of preparing a Cu-BTC comprising the steps of:
a) preparing a reaction mixture by dissolving benzene-1,3,5-tricarboxylate (BTC), dimethylformamide (DMF) and alcohol;
b) dropwise adding of reaction mixture of step a) into the solution of Cu(NO3)2.3H2O and water with stirring for 10 to 15 minutes at room temperature to obtain a homogenous mixture;
c) sonicating the homogenous mixture obtained in step b) and heating to obtain the precipitates; and
d) filtering, collecting, drying the precipitates and vacuum activating to obtain the (Cu-BTC).

11. A process for gas adsorption, said process characterized in employing a porous bimetallic metal-organic frameworks composite as defined in claims 1-3, and wherein the gas is selected from the group comprising hydrogen, methane, and carbon dioxide.

12. The process as claimed in claim 11, wherein the gas is hydrogen and is adsorbed in a range of 0.1 to 0.7 wt % at a temperature of 298 K and at a pressure of 100 bar.

13. The process as claimed in claim 11, wherein the gas is methane and is adsorbed in a range of 100 to 300 cc/g at a temperature of 298 K and at a pressure of 65 bar.

14. The process as claimed in claim 11, wherein the gas is carbon dioxide and is adsorbed in a range of 250 cc/g to 450 cc/g at a temperature of 298 K and at a pressure of 25 bar.